The present invention relates generally to processes for purification of hydrocarbons, and more particularly to adsorption processes using sorbents selective to sulfur compounds and to aromatic compounds.
Petroleum is an extremely complex mixture and consists predominantly of hydrocarbons, as well as compounds containing nitrogen, oxygen, and sulfur. Most petroleums also contain minor amounts of nickel and vanadium. The chemical and physical properties of petroleum vary considerably because of the variations in composition.
The ultimate analysis (elemental composition) of petroleum tends to vary over relatively narrow limits—carbon: 83.0 to 87.0 percent; hydrogen: 10.0 to 14.0 percent; nitrogen: 0.1 to 1.5 percent; oxygen: 0.1 to 1.5 percent; sulfur: 0.1 to 5.0 percent; metals (nickel plus vanadium): 10 to 500 ppm.
Crude oils are seldom used as fuel because they are more valuable when refined to petroleum products. Distillation separates the crude oil into fractions equivalent in boiling range to gasoline, kerosene, gas oil, lubricating oil, and residual. Thermal or catalytic cracking is used to convert kerosene, gas oil, or residual to gasoline, lower-boiling fractions, and a residual coke. Petrochemical intermediates such as ethylene and propylene are primarily produced by the thermal cracking of light hydrocarbon feedstocks in the presence of steam. Catalytic reforming, isomerization, alkylation, polymerization, hydrogenation, and combinations of these catalytic processes are used to upgrade the various refinery intermediates into improved gasoline stocks or distillates. The major finished products are usually blends of a number of stocks, plus additives.
Gasoline is a complex mixture of hydrocarbons that distills within the range 100 to 400° F. Commercial gasolines are blends of straight-run, cracked, reformed, and natural gasolines. Straight-run gasoline is recovered from crude petroleum by distillation and contains a large proportion of normal hydrocarbons of the paraffin series. Cracked gasoline is manufactured by heating crude-petroleum distillation fractions or residues under pressure, or by heating with or without pressure in the presence of a catalyst. Heavier hydrocarbons are broken into smaller molecules, some of which distill in the gasoline range. Reformed gasoline is made by passing gasoline fractions over catalysts in such a manner that low-octane-number hydrocarbons are molecularly rearranged to high-octane-number components. Many of the catalysts use platinum and other metals deposited on a silica and/or alumina support. Natural gasoline is obtained from natural gas by liquefying those constituents which boil in the gasoline range either by compression and cooling or by absorption in oil.
Removal of the sulfur-containing compounds is an important operation in petroleum refining, and is achieved by catalytic processes at elevated temperatures and pressures. See, Farrauto, R. J.; Bartholomew, C. H. Fundamentals of Industrial Catalytic Processes, Chapman and Hall, New York, 1997. The hydrodesulfurization (HDS) process is efficient in removing thiols and sulfides, but much less effective for heterocyclic diunsaturated sulfur compounds, such as thiophenes and thiophene compounds/derivatives (e.g. benzothiophene and dibenzothiophene).
New legislation will require substantial reductions in the sulfur content of transportation fuels. For example, the new U.S. Environmental Protection Agency (EPA) sulfur standards require that the sulfur content in gasoline and diesel fuels for on-board transportation will be 30 ppm and 15 ppm, respectively, drastically decreased from the current acceptable levels of several hundred ppm. Faced with the severely high costs of compliance, a surprising number of refiners are seriously considering reducing or eliminating production of on-board fuels. See Parkinson, G., “Diesel Desulfurization Puts Refiners in a Quandary,” Chemical Engineering, 2001, February issue, 37.
During the last decade, there have been several published accounts on using adsorption for liquid fuel desulfurization. Commercially available sorbents (i.e., zeolites, activated carbon and activated alumina) were used in all of these studies. Weitkamp et al. reported that thiophene adsorbed more selectively than benzene on ZSM-5 zeolite. See Weitkamp, J.; Schwark, M.; Ernest, S. “Removal of Thiophene Impurities from Benzene by Selective Adsorption in Zeolite ZSM-5,” J. Chem. Soc. Chem. Commun., 1991, 1133. Without being bound to any theory, it is believed that this is because thiophene (C4H4S, also known as thiofuran) has a higher dipole moment (0.55 debye) than benzene (non-polar), although their polarizabilities are similar. Based on this study, King et al. studied selective adsorption of thiophene, methyl- and dimethyl-thiophenes (all with one ring) over toluene and p-xylene, also using ZSM-5. See King, D. L.; Faz, C.; Flynn, T. “Desulfurization of Gasoline Feedstocks for Application in Fuel Reforming,” SAE Paper 2000-01-0002, Soc. Automotive Engineers, Detroit, 2000. They showed that thiophene was more selectively adsorbed, both based on fixed bed breakthrough experiments. However, the capacities for thiophene were unfortunately quite low (only 1–2% wt. adsorbed at 1% thiophene concentration). Both vapor phase and liquid phase breakthrough experiments were done in these studies, and the results from two phases were consistent.
The pore dimensions of ZSM-5 are 5.2–5.6 Å. Hence, organic sulfur compounds with more than one ring will be sterically hindered or excluded. Zeolites with larger pores, as well as larger pore volumes, would appear to be more desirable than ZSM-5 as the selective sorbents. Indeed, results of Salem and Hamid indicated that 13× zeolite as well as activated carbon had much higher sorption capacities for sulfur compounds. See Salem, A. S. H.; Hamid, H. S. “Removal of Sulfur Compounds from Naphtha Solutions Using Solid Adsorbents,” Chem. Eng. Tech., 1997, 20, 342. Based on the data of Salem and Hamid, Id., the capacity for sulfur compounds by 13× zeolite was approximately an order of magnitude higher than that of ZSM-5, when compared with the data of King et al. (cited above) extrapolated to the same conditions.
Activated alumina (Alcoa Selexsorb) has been used in an adsorption process by Irvine. See, for example, U.S. Pat. No. 5,730,860 issued to R. L. Irvine in 1998, entitled “Process for Desulfurizing Gasoline and Hydrocarbon Feedstocks.”
No direct comparison has been made among these commercial sorbents. Their experiments were mostly done in fixed bed adsorbers, by measuring the breakthrough capacities. Based on the literature, the large pore zeolites (NaX or NaY) are about the same as activated carbon and alumina, in terms of adsorption of thiophene.
Essentially all industrial adsorption processes are based on van der Waals interactions between the sorbate and the sorbent. Chemical bonds have yet to be exploited.
Thus, it would be desirable to provide an adsorption process for selectively removing sulfur compounds from liquid fuels at ambient temperature and pressure, thereby advantageously leading to a major advance in petroleum refining. It would further be desirable to provide highly selective sorbents for this process, thereby overcoming the drawbacks of current commercial sorbents, which are not desirable for this application.